[SalesForce] Default value for an attribute of type Object

I am trying to default Site_Name__c for an attribute "SEComp" from other attribute.

It is giving compilation error as Cannot mix expression and literal string in attribute value, try rewriting like {!'foo' + v.bar}

It appears I am getting stuck defaulting from other attribute but default just works fine if I say 'Site_Name__c': 'Abc'

What is the right way of defaulting for this?

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,force:hasSObjectName" controller="KF_CompanyLookupController">
<ltng:require styles="/resource/SLDS231/assets/styles/salesforce-lightning-design-system.css"/>
<aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
<aura:attribute name="SEComp" type="Searcher_Express_Company__c"
                default="{'sobjectType' : 'Searcher_Express_Company__c',
                           'Site_Name__c': {!v.defaultSite.siteName}}"/>
<aura:attribute name="defaultSite" type="KF_CompanyLookupController.Site"/> 

helper class

getUserSite : function(component){
        var action = component.get("c.getdefaultSite");
        action.setCallback(this,function(response){
            var state = response.getState();
            if (state === "SUCCESS") {
                component.set("v.defaultSite",response.getReturnValue());
            }
            else if (state === "ERROR") {
                var errors = response.getError();
                if (errors) {
                    if (errors[0] && errors[0].message) {
                        console.log("Error message: " +
                                 errors[0].message);
                    }
                } else {
                    console.log("Unknown error");
                }
            }
        });
        $A.enqueueAction(action);
}

Apex controller

public class Site {
    @AuraEnabled
    public String siteName {get;set;}
    @AuraEnabled
    public String siteID {get;set;}
    public Site (){
      this.siteName = siteName;
      this.siteID   = siteID;
    }
}   

@AuraEnabled
public static Site getdefaultSite(){
    Site dSite = new Site();
    User currentUser = [select Division, Country, Office__c from User where id = :UserInfo.getUserId()];
    system.debug('currentUser - Division: ' + currentUser.Division + ' Country: ' + currentUser.Country + ' Office__c: ' + currentUser.Office__c);
    if (currentUser.Division == 'Futurestep') {
        // 1. check to see if user is FS
      dSite.siteID = '50';
      dSite.siteName = 'Futurestep';
    } else if (currentUser.Country == 'United States' || currentUser.Country == 'Canada' || currentUser.Country == 'Brazil') {
    // 2. check to see if user is in NA
      dSite.siteID = '0';
      dSite.siteName = 'North America';
    } else {
      String userOffice = currentUser.Office__c;
      if (!String.isEmpty(userOffice)) {
        List<Business_Rule_Processor__c> defaultSites = [Select Site_Name__c,Site_Id__c from Business_Rule_Processor__c
                                                          where RecordType.Name = 'Default Site'
                                                          AND Type__c = 'Default Site'
                                                          AND Active__c = true
                                                          AND Office__c includes (:userOffice)];
        if (defaultSites.size() > 0) {
          string defaultSiteName = defaultSites[0].Site_Name__c;
          String defaultSiteId = defaultSites[0].Site_Id__c;
          if (!String.isEmpty(defaultSiteName)) {
            dSite.siteName = defaultSiteName;
            dSite.siteID = defaultSiteId;
          }
        }
      }
    }
  System.debug('dSite value is'+dSite);
  return dSite;
}